The hamster papovavirus (HaPV) was originally isolated from skin epitheliomas originating from hair follicle epithelial cells in Syrian hamsters. The HaPV virions are found in the keratinized layer of the epithelium from infected animals, but are not found in the basal layers. Thus, the maturation of this virus is limited to terminally differentiated keratinocytes thereby resembling the tissue-specific tropism of the papillomaviruses. However, the morphology of HaPV virions, the DNA sequence of the HaPV genome, and the genetic organization of the HaPV genome clearly show that this virus is a member of the polyomavirus family. In contrast with other members of the polyomaviruses and papillomaviruses, HaPV injection into newborn hamsters produces rapid and acute lymphomas and leukemias which are thought to be of T-cell origin. Thymectomy of the animals severely reduces the incidence of this disease but results in formation of sarcomas at the site of injection. Thus, the HaPV is capable of inducing tumors of lymphoid, mesenchymal, and epithelial origin in its natural host. The viral genes responsible for this broad tumor potential and the control of the expression of these genes is currently unknown.